Sailcraft: Uses of the Beam

byPaul GilsteronAugust 22, 2013

I’m always on the lookout for practical ways to use solar sails. We think long-term here and interstellar flight is the topic, but the other side of that coin is that we need to see incremental progress made that builds toward a significant human presence in space. Creating the installations to send powerful microwave or laser beams to interstellar sailships will involve mastering all kinds of needed objectives starting with getting large payloads to low-Earth orbit cheaply and building plentiful expertise in moving supplies at interplanetary distances.

Sails can go to work for us here, and before we get to that level, we can chart a path of development with clear, practical uses that companies and governments can support. Yesterday I mentioned Les Johnson’s talk at Starship Congress in Dallas, in which he described ongoing sail efforts and noted that NASA’s Sunjammer sail was partly sponsored by the National Oceanic and Atmospheric Administration. Early warning for solar storms is a practical outcome and one space agencies should be able to use to gain funding. Meanwhile, we can watch private attempts like the Planetary Society’s Lightsail-1 and hope they can find their way to a launch manifest, which would continue to validate the model of commercial engagement.

Image: I knew I had at least one good shot of Les Johnson from the conference, but I couldn’t find it yesterday. A quick search of my iPhone pulled it out. The shot above was snapped at the Hilton Anatole’s top-floor restaurant the evening of the first day in Dallas.

One other kind of mission that Johnson mentioned, a sail rendezvous with a near-Earth asteroid, is certainly viable. There are various ways to operate around asteroids, but the sail offers you the option of visiting multiple objects, performing studies as needed and moving to the next target. Targeted missions by chemical or even ion rockets don’t have an open-ended target list, but the sail can rely on ever-present solar photons to keep it operational, giving us deeper insights into objects we have now begun to catalogue extensively for science and for planetary safety.

The Beam and the Near Miss

When it comes to putting a beam on a sailcraft to keep it operational even at distances greater than 5 AU — or to provide a huge velocity boost for closer-in missions between the Earth and Mars — we also need to be thinking about practicality. A Mars colony or an aggregation of asteroid miners in need of supplies could use the services of beamed sailships. Greg and Jim Benford have studied the effects of desorption of polymer layers on a sail, an effect caused by heating the sail with the beam that provides accelerations greater than those achieved by photons alone. Mars travel times of about a month can result from proper use of the beam.

We can imagine a network of sail-based supply ships as we spread out into the Solar System, an era in which we master solar sails and their beamed sailship counterparts while doing necessary work. Also intriguing at Starship Congress was another angle on the practicality of beaming, this one from Philip Lubin who, with co-researcher Gary Hughes, has been studying ways of dealing with asteroids on problematic trajectories. Lubin (UC-Santa Barbara) described what we might consider a directed energy orbital defense system using lasers fed by solar energy. It could be a major addition to our toolkit of asteroid deflection options.

Ironically enough, as Lubin told the crowd in Dallas, the white paper describing his DE-STAR system was released the day before the spectacular fireball over Chelyabinsk and the near-miss from asteroid 2012 DA14 reminded the world that asteroids can be dangerous. Lubin pointed out that 35,000 impacts from various objects have occurred in the last 4000 years, with about 100 tons of material entering the atmosphere on a typical day. Most of this is obviously not causing carnage, but we’re charting the orbits of large objects to make sure nothing threatens the planet with an extinction level event. We’d obviously like to get to where we could rule out Tunguska-class impacts as well, which are large enough to take out an entire city.

DE-STAR (Directed Energy Solar Targeting of Asteroids and exploRation) is a way to deal with such impactors. It would use a massive phased array of lasers to break up or evaporate the objects. The Santa Barbara team has calculated the requirements for a range of DE-STAR systems beginning with a desktop device and extending to 10-kilometer arrays and beyond. Their DE-STAR 2 is envisioned as a 100-meter system that could nudge asteroids or comets enough to avoid an impact.

But a 10-kilometer DE-STAR 4 could punch an asteroid with 1.4 megatons of energy per day, obliterating a 500-meter asteroid over the course of a year. Lubin described how space photovoltaics could be applied to create a beam of coherent focused light that would produce a spot size of about 30 meters at a distance of 1 astronomical unit. The beam would be applied to the asteroid when it was still distant, perhaps 1 AU out. And while totally evaporating an incoming object is feasible, it’s hardly necessary. The goal is simply to deflect it.

A phased array of lasers powered by the Sun scales up nicely in Lubin’s calculations from basic components that already exist to larger space-based installations with which we’ll have to develop expertise. Here again, though, there is a path of development and a clear rationale, planetary defense. And I’m sure you can see where I’m going with this: DE-STAR can produce the beam needed to drive a sailcraft to interstellar velocities. Lubin told the Dallas audience that a much larger DE-STAR 6 system could bring a 100 kilogram sailcraft up to 1200 kilometers per second over a range of 1 AU, while pushing it to 2 percent of the speed of light by the time it reached the edge of the Solar System.

Interestingly enough, there are a few SETI ramifications here as well. A DE-STAR 4 system is so powerful that when directed at another star it would be visible as the brightest star in the sky up to 1000 light years away. Those who think in terms of sending messages to extraterrestrial civilizations — and the subject of METI, always controversial, came up in Dallas, about which more later — could modulate a laser. Lubin’s is one way of creating the kind of interstellar beacon many of us think should not be built until we can reach a rational consensus about what the aims of such messages would be and what the possible risks of sending might involve.

The Triumph of the Small

I don’t want to close today without mentioning nanotechnology in sail terms, because as Eric Malroy made clear to the Starship Congress audience, mass is a critical concern as we move toward making larger sail structures. We want to push these things up to interstellar speeds, but the stronger the beam we put on the sail, the higher its temperature. So we’re looking for the right kind of materials to withstand the heat even while reaching incredibly low areal densities, at which point acceleration, in Malroy’s terms, ‘goes through the roof.’

A lot of work has already gone into sail materials, much of it by Geoffrey Landis (NASA GRC), who took Robert Forward’s aluminum sail structures and re-examined them. Niobium, beryllium and transparent films of dielectric (non-conducting) materials like silicon carbide, zirconia and diamond all fell under his attention as he searched for ways to bring sailships to higher accelerations while using smaller sails. Beryllium was an early front-runner but Landis went on to talk about sails made out of diamond-like carbon — a material much like diamond — that could be assembled in space with a plastic substrate. The beauty of this is that you can reach cruise velocity while still close to the laser source, thus using smaller sails and smaller lasers.

Malroy (NASA JSC) noted the possibility of using nanotechnology to print computing and sensor devices directly onto the sail, significantly reducing mass. The idea reminded me of Forward’s Starwisp, a microwave-driven sail that contained sensors at the various junctions of the spider web-like structure, so that instead of pulling a payload along behind, the sail and the payload were the same structure. Lenses and nano-transmitters could return imagery from distant solar systems using these techniques, and Malroy also described swarms of miniature beamed sails of the kind first described by Jordin Kare. In any event, he believes that by properly deploying nanotechnologies, we can reduce interstellar sail mass by 70 percent.

If these concepts intrigue you, you can read more about Starwisp in Remembering Starwisp, though the archives contain a number of other Centauri Dreams posts that get into the concept. Jordin Kare’s ‘SailBeam’ micro-sails are described in Interstellar Propulsion Exotica. But we won’t get to the beamers needed to drive such spacecraft until we can show a sustainable path of development that begins with practical applications today. That’s why the idea of experimenting with phased arrays for asteroid mitigation appeals to me. It’s laboratory work we can perform now with implications that could take us out of the Solar System.

Tomorrow I’ll be looking at an intriguing concept for building an interstellar infrastructure that was presented at Starship Congress. For now, though, let me mention that the paper on sail desorption mentioned above is Gregory Benford and James Benford, “Power-Beaming Concepts for Future Deep Space Exploration,” in the Journal of the British Interplanetary Society Vol. 59 No. 3/4 (March/April 2006), pp. 104-107. Jim Benford didn’t get into the desorption issue in his Dallas talk, but it’s provocative and gives sails an added kick.

And to close, this final photo, showing my son Miles and science fiction writer Oz Monroe, who flew in from the West Coast for Starship Congress. We had a great dinner sitting around the Hilton Anatole’s pool with Sonny White along with Rob Adams (NASA MSFC) and his wife Tia. The cheeseburger you see here is Sonny’s.

Comments on this entry are closed.

Alex TolleyAugust 22, 2013, 10:43

Asteroid mitigation for planetary defense should provide access to DoD funding. It might well be possible to piggyback solar sail faunding to do the reconnaissance/exploration/science on several potential targets to determine their suitability for laser vaporization. (Speaking of which, does that actually work well on a tumbling body?).

I’m pleased that there seems to be some more serious thoughts about commercial development paths to interstellar flight, rather than just grandiose proof of concept ideas.

This article and that of 19 Aug touch on heat damage to the lightsail from the propulsion beam.
There are great strides being made in photovoltaics. Photovoltaic materials are becoming thinner, lighter, flexible, and photovoltaic effects can be produced by nanostructures composed of various materials including graphene and carbon nanotubes. Some materials apparently become more, not less, efficient with rising temperature.
Suppose a photovoltaic film were deposited or painted onto a lightsail, or incorporated into the structure of the sail itself. The waste energy of the propulsion beam would largely be converted into useable power rather than destructive heat. If the frequency of the beam and the spectrum efficiency of the photovoltaic process were coordinated, the heat gain would be minimized and the power maximized. Present research is naturally directed toward the production of electricity from the earth-surface solar spectrum, but various materials and structures have been shown to create a photovoltaic effect at other frequencies.
While power is necessary for observation, navigation, communications, and other spacecraft uses, the great majority of it would be available for propulsion. An ion engine would demand onboard propellant whose mass would have to be accelerated and which would eventually be exhausted, though a pure photon drive would not have these problems.
There is also the issue of decelerating an interstellar lightcraft at the target system. If the energy from the beam could be accumulated in a high density storage device (gluon capacitor or some such future magic) it could power an ion thruster using the no longer needed lightsail as propellant.

Yes, very small scale lithography is a real technology with enormous benefits. I could not agree more that reducing payload mass of sail driven probes is one of the best enablers of enhanced performance.

PS: I have come to despise the term “Nanotechnology” (i.e. K. Eric Drexler) as too many people identify the term with the well debunked 1980s SciFi concept of molecular assemblers.

With the advent of 3D printing large sails could be manufactured in space, aside of, lets say, making aviable all the resources in the asteroid belt to Earth.

Its interesting the argument of SETI was brought up. Somehow that Sirius red controversity issue reminds me about “The Mote in God’s Eye”, best illustrated in Homer’s IIliad:

“Sirius rises late in the dark, liquid sky
On summer nights, star of stars,
Orion’s Dog they call it, brightest
Of all, but an evil portent, bringing heat
And fevers to suffering humanity.”

I can’t help thinking along the Mote’s line: “…they got lasers big enough to light up the night sky.” Certainly a few of Van Danikens stories about indian tribes possesing advanced cosmological knowledge and tribal dances in strawsuits don’t really make those thoughts any more comfortable.

But… this technology is far to useful to be ignored, even on a stellar scale. It is certainly the way to go.

@Swage – great link. (And just the data I really wanted to see about Myrabo’s lightcraft but was missing from his book.)

So the report’s conclusion is that beamed power is very feasible for a range of uses with the caveat that infrastructure costs (beam facility) is the main cost issue. Interestingly they they see frequent, small payload cubesats launches as a good developmental route.

All good news for moving down the beamed sails approach for deep space missions.

Maybe a thick dust cloud was between Sirius and Earth at the time converting Sirius’s star systems light to red.

A big problem with lightsails is encountered interstellar gas on the way as pointed out many times here before. If we used the large area of the sail for the acceleration phase and then made it smaller for the rest of the mission (roll it up and point it towards the target) dust and gas would be much less of a problem.

The usual explaination you get for the red controversity is atmospheric effects. There are quite few historic figures describing Sirius as red, among them Ptolemy (a greek astronomer, for those who can’t connect with the name). It seems strange an astronomer would be deluded by atmospheric effects, especially him. Homer’s Iliad describing the days the star rose as searing hot. This doesn’t correlate well to an obstructive dust cloud. This is refered to as “dog days”, the hottest days (Sirius is of course the “dog star”), an expression still in use today. Stellar evolution, as far as we understand it can also be ruled out since such changes are not supposed to happen so rapidly. And then there are myths about visitors from african tribes, describing the sirius star system apparently in great detail, despite obviously lacking the tools to make the neccesary observations, recorded 1930 by Marcel Griaule, a French anthropologist. This is usually attributed to poorly executed questioning methods. All in all these are small inconciesties, not really worth discussing. What makes me suspicous is how good it all adds up. Most likely its nothing, inconcistencies in observation and recording, but i like it as a reminder not to expect we are the first civilization using technology to leave our cradle. It gives a nice idea about the kind of headstart we can expect if we should make contact.

A phased array of lasers powered by the Sun scales up nicely in Lubin’s calculations from basic components that already exist to larger space-based installations with which we’ll have to develop expertise.

I am curious which “phased array of lasers” technology the DE-STAR concept is built on. In my limited understanding there is no such thing, with optical phased arrays deep in the fundamental research stage. See, for example, the “Largest-ever optical phased array” described here: http://www.rle.mit.edu/watts-releases-new-paper/, which measures less than a millimeter across. It also does not include any lasers, so it is unclear how suitable it would be for beam propulsion.

Since DE-STAR is claimed to be based on existing technology, it would be nice to have a real reference to the “phased array of lasers” technology that was used as the basis for the proposal. Did I miss it?

Jim Benford didn’t get into the desorption issue in his Dallas talk, but it’s provocative and gives sails an added kick.

Perhaps it is useful to note that the desorption issue reintroduces the one thing into sail technology that we most want to avoid: Carried reaction mass, and with it the dreaded rocket equation. Isn’t avoiding these albatrosses why we want to do sails in the first place, and what advantage over regular rockets remains if you add them back in?

One thing (out of many) that intrigued me about Lubin’s excellent talk is the possibility of using GROUND based lasers, as in his systems, to launch relatively low mass payloads into LEO using laser thermal rockets . The idea is an old one, and has been explored by many, including the Seattle based company LaserMotive. LaserMotive has had considerable success in winning a NASA power-beaming contest and has kept a small uav in flight for 48 hours using laser beamed power. Even a relatively small DE-STAR system might be very capable of powering small, single-stage to orbit craft. And, of course, it would also make a pretty decent point-defense anti-ballistic missile system (and so might draw more DoD funding). In fact, the politics of building such a system on the ground are probably much more easily navigated than one in earth orbit, at least for the near future. This brings to mind Greg Benford’s excellent short story, “To the Storming Gulf”.

Apparently words for colors take time to invent. Even te Greeks had few colors to describe objects. Red is often the first color that is given a word. See Guy Deutscher’s book: “Through the Language Glass: Why the World Looks Different in Other Languages”

An exotic new form of propulsion could send a spacecraft to Uranus in the time Galileo took to get to Jupiter less than half as far away, say rocket scientists.

Humankind has sent only handful of spacecraft to the outer Solar System beyond the asteroid belt. There were the Pioneers and Voyagers that left Earth in the 1970s. The Galileo mission headed to Jupiter in 1989 and Cassini-Huygens to Saturn in 1997. Finally, the New Horizons mission left Earth in 2006 and is currently heading towards Pluto and the Kuiper Belt.

One problem with these missions is the sheer time and cost involved. Galileo took 6 years to reach Jupiter and cost about $1.6 billion while Cassini-Huygens took 7 years to get to Saturn and cost almost as much.

Now, a Finnish-led team is proposing a mission to Uranus powered by an exotic new form of propulsion that is currently being tested in Earth orbit. This propulsion is solar-powered and so does not require on board propellant. And it can send a probe to Uranus in about the same time that Galileo took to get to Jupiter which is less than half as far away. But the costs of such a mission are not yet clear.

One problem for probes visiting the outer Solar System is in generating the velocity necessary to get there, against the pull of the Sun’s gravity. The original plan for the Galileo mission, for example, was to use the space shuttle to place the probe and its booster rocket in Earth orbit. The booster was designed to send the probe directly to Jupiter in less than two years.

But in the aftermath of the Challenger disaster, NASA decided that it would not be sensible to place an unlit rocket in the cargo hold of the shuttle. And since no other rocket could lift Galileo and its booster, NASA had to find another way to get there. Hence the slingshot approach.

The proposed Uranus mission takes an entirely different approach based on the concept of an electric sail or E-sail, which was proposed by Finnish engineer Pekka Janhunen in 2006 (who is also the lead for the new Uranus proposal). An E-sail is significantly different from a conventional solar sail, which generates thrust from the pressure of photons hitting the sail.

By contrast, the E-sail relies on charged particles such as protons and alpha particles in the solar wind. The idea is to generate an electric field around the spacecraft which deflects these ionised particles and generates a force that accelerates the craft throughout its journey.

The sail consists of a set of conducting wires that extend radially from the spacecraft like spokes on a wheel. The electric field is generated using solar power. And with 540 Watts, the sail should generate about 0.5 Newtons accelerating the craft by about 1 mm/s^2.

That should produce a velocity of about 20 km/s by the time it reaches Uranus giving a journey time of about 6 years.

The craft itself is designed in three parts. The first is the E-sail module with solar panels and tether reels to extend the wires. The second is the main body of the craft with chemical thrusters for making trajectory adjustments en route and while close to Uranus, as well as communications equipment for contact with Earth.

The final part is an entry module that is released in to the atmosphere of Uranus, where it makes various scientific measurements for transmission back to Earth via the main craft which acts as a relay.

Janhunen and co say this design would also be suitable for journeys to other gas giants with minor modifications.

That’s an ambitious idea, not least because the E-Sail idea has never been tried on a scale anything like this. One small satellite called ESTCube-1 is currently testing the idea and the European Union has an ongoing research project to test it further. Another Finnish satellite will test the principle in more detail this year but more work will surely be needed for a mission of this type.

Nevertheless, the advantages of E-sails over gravitational slingshots are clear. In addition to being quicker, they can also be launched at almost any time with only minor variations in travel time. By contrast, slingshots can only go when the gravitational gods are in alignment.

What Janhunen and co do not discuss is the cost of such a mission, which to be fair is hard to pin down at this stage of a design. The advantage of a Cassini or Galileo type mission is that these craft operate for years around their target planets sending back huge troves of data albeit at massive cost. The disadvantage is that everything is lost if the spacecraft is lost somehow.

By contrast the E-sail mission sends back a few minutes of data from its fiery entry into the atmosphere. That’s surely valuable but would have to be significantly cheaper to justify.

So a good understanding of the relative costs of these kinds of missions will be crucial in determining whether E-Sails have a future in the exploration of the outer Solar System.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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